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From: Stephen Hemminger <stephen@networkplumber.org>
Subject: Re: [PATCH v2 10/10] kni: add API to set link status on
 kernel interface
Date: Wed, 29 Aug 2018 16:10:43 -0700
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Cc: Ferruh Yigit <ferruh.yigit@intel.com>, dev@dpdk.org
To: Dan Gora <dg@adax.com>
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On Wed, 29 Aug 2018 19:41:23 -0300
Dan Gora <dg@adax.com> wrote:

> On Wed, Aug 29, 2018 at 7:12 PM, Dan Gora <dg@adax.com> wrote:
> > On Wed, Aug 29, 2018 at 7:00 PM, Stephen Hemminger
> > <stephen@networkplumber.org> wrote:  
> >>> >> Add a new API function to KNI, rte_kni_update_link() to allow DPDK
> >>> >> applications to update the link state for the KNI network interfaces
> >>> >> in the linux kernel.
> >>> >>
> >>> >> Note that the default carrier state is set to off when the interface
> >>> >> is opened.
> >>> >>
> >>> >> Signed-off-by: Dan Gora <dg@adax.com>  
> >>> >
> >>> > Do you really need a special ioctl for this?
> >>> > There is already ability to set link state via sysfs or netlink.  
> >>>
> >>> I think yes.. AFAIK sysfs does not constitute a stable API;  
> >>
> >> It is a stable API on Linux.  
> >  
> 
> Actually this does not seem to be completely true...
> 
> From Documentation/admin-guide/sysfs-rules.rst:
> 
> Rules on how to access information in sysfs
> ===========================================
> 
> The kernel-exported sysfs exports internal kernel implementation details
> and depends on internal kernel structures and layout. It is agreed upon
> by the kernel developers that the Linux kernel does not provide a stable
> internal API. Therefore, there are aspects of the sysfs interface that
> may not be stable across kernel releases.
> 
> <snip>
> 
> - devices are only "devices"
>     There is no such thing like class-, bus-, physical devices,
>     interfaces, and such that you can rely on in userspace. Everything is
>     just simply a "device". Class-, bus-, physical, ... types are just
>     kernel implementation details which should not be expected by
>     applications that look for devices in sysfs.
> 
>     The properties of a device are:
> 
>     - devpath (``/devices/pci0000:00/0000:00:1d.1/usb2/2-2/2-2:1.0``)
> <snip>
> 
>     - kernel name (``sda``, ``tty``, ``0000:00:1f.2``, ...)
> <snip>
> 
>     - subsystem (``block``, ``tty``, ``pci``, ...)
> <snip>
> 
>     - driver (``tg3``, ``ata_piix``, ``uhci_hcd``)
> <snip>
> 
>     - attributes
> <snip>
> 
>     Everything else is just a kernel driver-core implementation detail
>     that should not be assumed to be stable across kernel releases.

Network device sysfs is stable. No one ever got around to putting it in documentation
I wouldn't worry, once anything in /sys/class/net is added it is not going to change without major breakage in many many tools.